Wheat straw represents a large potential source of fiber; which is needed to manufacture paper where wood supply is scarce or expensive. Generally, straw is burned as waste and this results in environmental damage as well as lost of a possible cash crop. Many countries around the world including Pakistan, China, Turkey, Egypt, Spain, India and others use straw pulp to support to manufacture paper because of its ready availability (as industrial waste) and unavailability of wood (lack of enough forests). Since the use of wheat straw requires pulping and bleaching steps, this adds significantly to damaging environment through hazardous effluent discharges that adversely affect the eco-system. Many inventions are thus directly towards reducing the effluent load without affecting consumer supplies but none has fulfilled the gap between the industry need and the requirements of keeping environment clean.
The goal in bleaching chemical pulps is to remove essentially the chromophoric groups (mostly the residual lignin) capable of absorbing visible light. Hypo-bleaching (using hypochlorites and acids), which is significantly damaging to environment, is the traditional approach still practiced because of its effectiveness to fully bleach the pulp at a low cost. However, the pulp quality deterioration and production of hazardous and persistent organochlorines are the major disadvantages in the use of hypochlorite and hypochlorous acid bleaching.
Concerns about the environment and health due to chlorine bleaching resulted in the development of elemental chlorine free (ECF) and totally chlorine free (TCF) bleaching processes. The ECF sequences involve the use of chlorine dioxide that helps reduce the adsorbable organically bound halogens (AOX) while reducing dioxin discharges in wastewaters. The U.S. Pat. No. 5,164,043 to Griggs describes the use of ClO2 as the delignifying and bleaching agent that selectively oxidizes lignin; however, the said invention remains difficult to handle, and exposes paper mill personnel to health hazard.
TCF bleaching involves oxidative degradation of color rendering groups in the pulp. Oxygen, ozone, peroxides and peracids are the most common TCF agents for pulp bleaching but none of the TCF agents is alone capable of bleaching the pulp with full brightness without compromising its properties. TCF bleaching is relatively friendlier to ecology as compared to other bleaching sequences reported so far.
During bleaching, chromophoric groups such as lignin must be degraded and washed away while celluloses are preserved to give structure and strength to the paper. A few studies have reported TCF bleaching sequences for different types of wheat straw pulp [Hedjazi, S. et al 2009; Niu, C. et al 2007 and Wang, H. et al 2003]. The availability of bleaching equipment (e.g., ozone generators) with complete workplace safety equipment and the cost of processing at the production (mill scale) are hurdles in the use of this process. Another practical approach for bleaching is the use of biological enzymes. A few studies on wheat straw recommend the use of enzymatic treatment prior to bleaching; however, the shelf life and cost remain the limitation factors in the complete adoption of this process. Also the enzymes alone have never been reported used to bleach the pulp with full brightness. The incorporation of enzymatic stage in a bleaching sequence is to selectively degrade the lignin contents and ultimately reduce the use of chlorinated or elemental chlorine free chemicals [Tolen et al., U.S. Pat. No. 7,368,036] or TCF sequence [Han, S. et al 2002 and Rancero, M. B. et al 2003].
The various processes of papermaking involve comparable steps except the choice of pulping and bleaching chemicals and their sequences for application in light of different composition of lingo-cellulosic contents of the raw materials used for paper making. The sequence of treatment developed for one type of fibrous pulp may not prove be applicable to other types, leaving a large unmet need for the development of new and improved bleaching strategies to treat different pulp varieties.
The present invention reports selective and efficient bleaching sequences for AS/AQ wheat straw pulp where the conventional bleaching agents have been utilized in such a way that the amounts used, the reacting conditions employed and the sequences applied proved the preservation and protection of every type of pulp and paper properties with very low effluent load.